Flash lamps have a wide range of medical and cosmetic, as well as industrial, applications. Flash lamps in intense pulse light (IPL), fluorescent pulsed light (FPL), fluorescent light, or laser light sources for medical and cosmetic application function predominantly through plasma surface (e.g., blackbody) emission of electromagnetic radiation. One reason for the predominance of plasma surface emission is the empirical observation and working hypothesis that the emissivity and electro-optical efficiency of a flash lamp increases with current density, which points to the conclusion that high current density is needed to meet the practical need for emission of sufficient electromagnetic radiation. Another reason for the predominance of plasma surface emission is the great success of such flash lamps in the field of laser pumps, where surface emission is desirable and beneficial, which has resulted in successful laser technology being directly carried over for medical and cosmetic use.
In flash lamp design, the working hypothesis that higher efficiency results from higher current density operation (e.g., current density sufficient to support substantially blue-shifted or at least some blackbody radiation), lead naturally to the working hypothesis that higher efficiency can also be achieved through use a flash lamp having a smaller bore. Because the blackbody emission occurs from the plasma surface, and because the plasma (e.g., the volume of gas plasma inside the lamp) is opaque to the emission, flash lamp bore size is generally minimized to maximize the surface area to volume ratio and to minimize the loss of electromagnetic radiation emitted from within the volume of gas plasma inside the lamp (e.g., electromagnetic radiation that is not transmitted through the opaque plasma).
FIG. 1 shows calculated spectral emissivity curves for a 5 mm bore diameter 450 Torr Xenon lamp at various current densities. FIG. 1 also shows a calculated spectral emissivity curve for a 3 mm bore diameter 450 Torr Xenon lamp for comparison. Therefore, FIG. 1 illustrates the hypothesis that (i) emissivity and electro-optical efficiency should increase with increasing current density and that (ii) a small bore size is generally desirable.
In practice, the fact that at least some area of plasma surface is needed for emission and the fact that the surface emitting plasma is opaque has also kept the arc-length to bore ratio of traditional flash lamps relatively large (e.g., greater than about 12-9) and bore sizes relatively small (e.g., less than about 4-7 mm). Such traditional designs employ relatively high current densities (e.g., greater than about 5,000 Amps/cm2) to drive the flash lamp to produce peak wavelengths suitable, for example, for pumping Nd:YAG lasers. Even with relatively high current density operation, practioners and designers of flash lamp-based devices still seek to increase the output of their devices. Various lamp configurations have traditionally been attempted to improve fluence. These configurations include: IPLs with lamps next to each other, IPLs with lamps on top of each other, and U-shaped lamps. Each of these lamp configurations is characterized by a relatively small bore size.
Such small bore, high current density designs have continued to be used and developed because they are well-suited for many commercially significant applications. One reason that small bore, high current density designs have been successful and are pervasive in the flash lamp market is that they produce large amounts of broad band and blue-green light. In fact, high current densities result in high temperatures, which can cause blue shift in the emission spectrum and which can cause the flash lamp to function as a black body emitter (e.g., through surface emission), which was deemed to be largely desirable, useful, and efficient for laser pumping. These empirical observations have been carried over to IPL and FPL devices for medical and cosmetic application, resulting in small bore, high current density flash lamps dominating IPL and FPL devices for medical and cosmetic fields.
Although it is known that a flash lamp can provide at least some electromagnetic radiation under low current densities (e.g., as shown in FIG. 1), such low current density operation has not been previously demonstrated for treating organic tissue. Generally, it was believed low current density operation of a flash lamp did not provide sufficient electromagnetic radiation to effect a treatment.
For example, not only did naked conventional flash lamps (e.g., less than about 4-7 mm bore diameter) emit little electromagnetic radiation, but the necessity for filters to remove harmful and non-therapeutic bands of electromagnetic radiation (e.g., UV light), further reduced the output of flash lamps operating at low current densities to the point where the lamps were too inefficient to effect treatment or were simply incapable of affecting any treatment at all. Furthermore, the prediction of flash lamp output based on structural design and operating parameters is notoriously unreliable. Therefore, the design and operation of flash lamps continues to be dominated by the empirical observations and practical experience that small-bore, high current density flash lamps are preferable and desirable.